One way to cope with the increasing demand for transmission of information on optical fiber links and to optimize the investment in optical fiber links is to use more efficiently the bandwidth available in one link in order to increase its capacity. A Wavelength Division Multiplexing (WDM) technology enables to increase the number of transmitted channels by adding more transmitters and receivers using different wavelengths. However, the characteristics in WDM system are limited by the bandwidth of the amplifiers inside the link and the wavelength dependency on active or passive components inside the link. Therefore, the practical usage of WDM system is limited to the S band (Short band), C band (Conventional band), or L band (Long band) in the optical spectrum.
Another way to increase the capacity of a link is to increase the Spectral Efficiency (SE) by using more efficient modulation formats for the transmitted information. This can be used in conjunction with WDM. Optical communication systems with transmission rates up to 10 Gb/s mainly utilizes On Off Keying (OOK) for modulation, where the information is coded on two amplitude levels of the lightwave signal. Besides, higher capacity systems utilize the modulation scheme based on Quadrature Phase Shift Keying (QPSK), which codes the information on four phase levels. Therefore, two binary bits can be coded per transmitted symbol. This is illustrated in the non patent literature 1 (NPL1). In this manner, the necessary bandwidth of the optical spectrum required to transmit information is used more efficiently.
The other way to increase even more the spectral efficiency in a channel, and therefore the link capacity, is to use Quadrature Amplitude Modulation (QAM), where symbols are coded on phase and amplitude levels, and are organized as a combination of multi-level amplitudes in quadrature phase. An example of QAM system is disclosed in the non patent literature 2 (NPL2). In NPL2, the modulation format is 16QAM, where the information is coded into 16 levels, that is, 4 binary bits per symbol. This enables to increase the spectral efficiency as compared to QPSK. Furthermore, in the non patent literature 3 (NPL3), the use of 512QAM is disclosed, where the information is coded into 512 levels, that is, 9 binary bits per symbol, and the spectral efficiency increases even more as compared to 16QAM. Therefore QAM format is an efficient way to increase link capacity.
As illustrated by NPL2 and NPL3, there is a trade off between the achievable Spectral Efficiency (SE) and the achievable transmission distance. Therefore, depending on the distance on the fiber link with signal being transmitted, it is advantageous to be able to select the index of QAM format, i.e. the number of modulated symbols on the constellation or in other words power-of-two of the number of binary bits coded on one symbol, to optimize this trade-off. The use of a transmitter, which can emit light modulated at different QAM index depending on its setting, is an economically effective way to optimize this trade off.
QAM format can be performed with an optical IQ modulator, sometimes called Cartesian modulator, vector modulator, Dual Parallel modulator or nested modulator depending on the sources. In an IQ modulator, the electric signals drive two independent Mach-Zehnder devices, which can be called children Mach-Zehnder Modulators (MZM). The children MZM modulate the phase and amplitude of the same optical carrier wave. The phase in one of their outputs is relatively delayed by 90 degrees before being recombined. The phase delay between the outputs of the children MZM can be called an angle of quadrature and is ideally 90 degrees, modulo 180 degrees. These IQ modulators are used in NPL2 and NPL3 for QAM format and also used in NPL1 for QPSK modulation. These IQ modulators offer an efficient and proven way to perform QAM format.
However, it is known that there is a drift of DC (Direct Current) bias in IQ modulator due to variation of the temperature or ageing of the device. There are three types of affected biases, that is, the DC biases of each of the two children MZM and DC bias used to set the angle at quadrature. This is already known about QPSK modulation and also known about QAM format if it uses a modulator having the same structure. Drifts in biases result in incorrectly setting the modulator, which causes a degradation of the transmitted signal, and therefore a degradation of the received signal quality or in worst cases the impossibility to decode the received signal. This trouble is likely to be revealed in the characterization tests of the modulator at the production stage or at the assembly stage of the transmitter in which it is used, and at each time the modulator is first used for modulation of data, that is, at each start-up or reset operation. The same trouble is also likely to occur during the operation of the modulator. These troubles are solved for OOK, Phase Shift Keying (PSK) modulation and QPSK by using Auto Bias Control (ABC) circuits, which controls the biases of the modulators and to compensate for the DC bias change. In this manner, ABC technology can manage the drift of DC bias both at start-up or reset, and during operation.
The non patent literature 4 (NPL4) discloses a scheme which can be used for ABC to control the 90 degree phase between the outputs of the Mach-Zehnder devices. It is based on minimizing the RF power spectrum of the modulated signal. The underlying principle is that the interferences between I and Q data components enhance the RF power spectrum, and therefore that the angle of quadrature can be controlled by minimizing the RF power spectrum. This scheme in conjunction with known methods used to control the DC biases of children MZM enables to control the DC biases of an IQ modulator for QPSK modulation.
In the patent literature 1 (PTL1), the same principle as that of NPL4 is used and moreover, a dither frequency is added to control the angle of quadrature by controlling monitored spectral components relative to the dither frequency. In addition, it also explicates ABC circuits based on dithering for the control of the DC bias of the Mach-Zehnder devices. In the same way as for NPL4, such a method is effective for QPSK. It enables to compensate for the bias changes during operation and before the start-up of the modulator for QPSK.